1. Field of the Invention
The present invention relates to a variable displacement compressor that changes its discharge displacement by adjusting the inclination of a swash plate. More particularly, the present invention relates to a variable displacement compressor that stops drawing in refrigerant gas from an external refrigerant circuit and circulates the residual gas therein when the inclination of its swash plate is minimum.
2. Description of the Related Art
Vehicles typically have compressors employed in air conditioning systems. A compressor having a controllable displacement is desirable for accurately controlling the interior air temperature to make the ride comfortable for the vehicle's passengers. There is a type of compressor that is provided with a swash plate, which is tiltably supported on a rotary shaft, cylinder bores, and reciprocal pistons, which are accommodated in the bores. The inclination of the swash plate is controlled based on the difference between the pressure in a crank chamber and the pressure in the cylinder bores. The stroke of each piston is varied by the inclination of the swash plate.
Such a compressor does not employ an electromagnetic clutch to selectively connect or disconnect the shaft of the compressor with an external drive source to transmit drive force. The external drive source is directly connected to the rotary shaft. This structure eliminates shocks that would otherwise be produced by the ON/OFF action of such a clutch. Such a compressor improves the riding comfort of the vehicle's passengers. The structure also reduces the overall weight of the refrigeration system and thus reduces the manufacturing cost.
In such a clutchless system, the compressor is operated even when cooling is not necessary. With such compressors, it is important that, when cooling is unnecessary, the discharge displacement be reduced as much as possible to prevent formation of frost in the evaporator. If cooling becomes unnecessary or if frost starts forming, the circulation of the refrigerant gas between the compressor and the external refrigerant circuit should be stopped. As shown in FIG. 12, a typical compressor has a shutter 150 that blocks the gas from an external refrigerant circuit (not shown) from flowing into a suction chamber 154. This stops the circulation of the refrigerant gas.
As shown in FIG. 12, the hollow cylindrical shutter 150 is slidably accommodated in a shutter chamber 152, which is defined in a cylinder block 151. The shutter 150 moves along the axis of a rotary shaft 156 in accordance with the inclination of a swash plate 157, which is supported by the drive shaft 156. A rear housing 158 is coupled to the rear end of the cylinder block 151 with a valve plate 159 provided in between. The rear housing 158 includes a suction chamber 154, a discharge chamber 160, and a suction passage 154. The suction chamber 153 is connected to the external refrigerant circuit. The suction passage 153 is communicated with the external refrigeration circuit via the shutter chamber 152. A positioning surface 155 is defined on the cylinder block 151 between the shutter chamber 152 and the suction passage 153.
A plurality of cylinder bores 163 extend through the cylinder block 151. A piston 161 is coupled to the swash plate 157 by a pair of shoes 162 and one piston 161 is accommodated in each bore 163. The swash plate 157 rotates integrally with the rotary shaft 156. The rotating movement of the swash plate 157 is converted to linear reciprocating movement of each piston 161 in the associated cylinder bore 163. The stroke of the pistons 161 corresponds to the inclination of the swash plate 157.
When the swash plate 157 is fully inclined with respect to the axis of the shaft 156, in which state the compressor displacement becomes maximal, the shutter 150 is moved to an opening position as shown by the solid lines in FIG. 12. The shutter 150 in the opening position enables communication between the suction passage 153 and the suction chamber 154. Therefore, as the piston 161 reciprocates, the refrigerant gas is drawn into each cylinder bore 163 from the external refrigeration circuit via the suction passage 153 and the suction chamber 154. The gas is then compressed in the cylinder bore 163. The compressed gas is discharged to the external refrigeration circuit via the discharge chamber 160.
As the inclination of the swash plate 157 becomes smaller from this state, the shutter 150 moves toward the positioning surface 155. When the inclination of the swash plate becomes minimal, causing the compressor displacement to be minimal, the shutter 150 abuts against the positioning surface 155 as shown by the double-dotted lines in FIG. 12. The abutment restricts the movement of the shutter 150 toward the positioning surface 155 and positions the shutter 150 at a closed position such that the shutter 150 disconnects the suction passage 153 from the suction chamber 154. Accordingly, the refrigerant gas stops flowing into the suction chamber 154 from the external refrigeration circuit. This stops the circulation of refrigerant gas between the external refrigeration circuit and the compressor.
In the above compressor, refrigerant gas is discharged from the cylinder bores 163 into the discharge chamber 160 and then drawn into the crank chamber 164 when the refrigerant gas in the external refrigerant circuit is hindered from flowing into the suction chamber 154. The refrigerant gas in the crank chamber 164 flows into the suction chamber 154 and is then drawn into each cylinder bore 163 during the suction stroke of the piston 161. In other words, a circulation passage is formed in the compressor when the flow of the refrigerant gas from the external refrigerant gas into the suction chamber 154 is stopped. The circulation passage, through which refrigerant gas circulates, is defined between the cylinder bores 163, discharge chamber 160, crank chamber 164, the suction chamber 154 and the cylinder bores 163. Refrigerant gas includes mist-like lubricant. The lubricant circulates through the circulation passage suspended in the refrigerant gas to lubricate various parts in the compressor.
A valve plate 159 has a suction port 165 and a discharge port 166. The plate 159 also includes a flapper type suction valve 167 and a flapper type discharge valve 168 for selectively opening and closing the ports 165 and 166. The flapper type valves 167, 168 close the ports 165, 166, respectively. Therefore, in order to open the ports 165, 166, the valves 167, 168 should be flexed against their elasticity. The mist-like lubricant in the refrigerant gas liquefies and adheres to the valves 167, 168 and also on the ports 165, 166 at sections surrounding the port valves, where the valves 167, 168 come into contact with the ports 165, 166, respectively. The liquefied lubricant adheres the valves 167, 168 to the valve plate 159 and makes it difficult to open the valves 167, 168.
When its inclination becomes small, the swash plate 157 moves toward the rear end of the compressor (right in FIG. 12) along the axis of the rotary shaft 156. The movement of the swash plate 157 pushes the shutter 150 toward the positioning surface 155 and the pistons 161 toward the rear end of the compressor. Therefore, the piston 161 moves relatively close to the valve plate 159 with a short stroke when the swash plate 157 is minimally inclined. In this state, if the valves 168, 167 adhere to the valve plate 159 and liquefied lubricant resides in the cylinder bore 163, the lubricant may not be discharged from the bores 163. The liquefied oil in each bore 163 also obstructs the piston 161 from moving close to the valve plate 159. This affects the movement of the swash plate 157 and hinders it from moving to its rear end position, where the inclination angle becomes minimal. Accordingly, the shutter 150 is hindered from moving to the closed position for disconnecting the suction passage 153 and the suction chamber 154. This causes the refrigerant gas in the external refrigerant circuit to leak into the suction chamber 154.
In this case, if refrigerant gas in the external refrigerant circuit is liquefied by a decrease in ambient temperature, the liquefied refrigerant flows into the compressor via the suction passage 153. The liquefied refrigerant washes away the lubricant inside the compressor. When operation of the compressor is resumed with a large displacement, the lubricant in the compressor in the liquefied refrigerant is drawn into the external refrigerant circuit. Thus, lubrication in the compressor becomes less than desirable. The lubricant also flows into an evaporator in the external refrigerant circuit and thereby decreases the cooling efficiency.